The instrumentation sensor art has for many years used gyroscopic rate of turn instruments having a spinning gyro. The gyro when forced to turn about an axis perpendicular to its spin axis, exerts a measurable couple force that is portional to the rate of turn being sensed. Such gyro instrumentation is generally expensive due to the precision involved in its design, construction and operation.
More recently, instrumentation sensors have been developed that require no rotating parts, but operate on a principal of vibration of constrained "masses" driven by an actuator. Such vibrating masses may take a number of different configurations such as reed members, piezoelectric bender elements, or electromagnetic members. When used in combination herein, the "mass" and its "actuator" will be referred to collectively as "actuator/mass".
The principle of operation of vibratory mass instrumentation sensors is fairly simple. If a mass is vibrated or maintained in oscillation in a straight line along which it is guided by some constraint, the oscillating mass will not apply any force (other than its own weight) in a direction transverse to the guide as long as the guide maintains a constant orientation in space. However, if the guide is forced to rotate about an axis at right angles to itself, the oscillating member will apply alternating or pulsating forces to the guide member, the average magnitude of which will be proportional to the angular velocity of the forced rotation. Such forces can be measured by such sensors as piezoelectric bender elements. The forces exerted by the oscillating mass on the sensor element causes measurable electrical potential signals to be developed on the faces of the sensor element, which signals can be measured and calibrated to the rate of turn of the sensor instrument.
The alternating or pulsating forces from the oscillating mass can also be measured by other techniques known in the art, several of which are discussed in more detail in U.S. Pat. No. 2,544,646 of Barnaby, which is incorporated herein by reference with respect to its applicable discussions of the general art of such vibrating constraint mass instrumentation sensors. Further discussion of background art and description of a vibrating mass sensor system that measures rate of turn about multiple axes, is disclosed in U.S. Pat. No. 3,842,681 to Mumme. Another description of a piezoelectric vibrating beam rate sensor instrument, written by the inventor hereof, which describes a preferred construction of a system suitable for incorporating the resonance drive circuit of this invention, is illustrated in FIG. 1 and is conceptually described in an article entitled "Piezoelectric Vibrating Beam Rate Gyro" published in the Navy Technical Disclosure Bulletin, Vol. IV, No. 6 June 1979.
Requisite to all vibrating mass sensor instrumentation systems as described above is a drive circuit for establishing and maintaining the vibrating or oscillatory state of the actuator/mass at an optimum level throughout the operative period of the instrument. Generally, in such instrumentation, it is most desirable to vibrate the actuator/mass at its natural resonance. At any point in time such natural resonance of the actuator/mass will occur at a fixed frequency, known as the resonant frequency of the mass. When the mass is vibrated at its resonant frequency, the mass provides its maximum measurable output signal for use by the instrument's sensor. If the amplitude of the actuator/mass oscillations can be maintained at a constant level throughout the period of operation of the instrument, high measurement accuracy can be maintained indefinitely by the instrument.
The primary deficiency of prior art actuator/mass drive circuits has been their inability to maintain the oscillation drive to the actuator/mass at its natural (or peak) resonance level with changes in the resonant frequency of the actuator/mass during operation. Changes of the resonant frequency are most often caused by such environmental factors as temperature variations to which the actuator/mass is subjected. Since the frequency at which the natural resonance of the actuator/mass occurs can vary considerably with changes in temperature, the oscillatory drive circuit for the actuator/mass must be able to instantaneously track such resonant frequency shifts and to simultaneously drive the actuator/mass at the new resonant frequency level, in order to maintain the output amplitude of the actuator/mass at a constant, maximum level. Failure of the drive circuitry to instantaneously track in real time, the changes in resonant frequency of the actuator/mass, can cause large amplitude variations of the oscillating actuator/mass, resulting in significant instrument output error.
Prior art resonance drive circuits have typically used tuned circuit elements such as oscillators and/or active filters, and are generally tuned to an initial or average resonant frequency of the actuator/mass. With such tuned circuits, however, if the resonant frequency of the driven mass changes due to temperature variations, age, or for other reasons, the tuning circuits must be retuned or the drive circuit physically reconfigured, in order to maintain oscillation of the actuator/mass at its natural (peak) resonance. Such problems associated with tuned circuits are further amplified by manufacturing and component tolerances inherent in the construction of such driver circuits.
Prior art drive circuits have also generally incorporated complex means for sensing the instantaneous resonant frequency and amplitude of the actuator/mass, thus further contributing to circuit complexity and possible source for error and inaccuracy. For example, prior art drive circuits have typically used diodes for envelope detection of the sensed resonance signal and a field-effect transistor to provide the signal limiting function. Both of such devices have temperature and age dependent voltage thresholds which directly contribute to degradation of the driver's ability to precisely maintain the resonance amplitude of the mass over any period of time or over the normal operative temperature range variations. In short, prior art driver circuits have not been of a quality to enable the vibrating mass instrumentation systems to achieve the accuracy or the time/temperature/age stability of the predecessor gyro/type instrumentation.
The present invention effectively addresses and overcomes most of the above-mentioned deficiencies of prior art actuator mass driver circuits. The driver circuit of this invention does not use any tuned circuits, which allows the driver to maintain resonance of the actuator/mass over a broad range of resonant frequencies, without the need to tune or to reconfigure the drive circuit. Accordingly, manufacturing tolerances and their effect upon the drive circuitry are minimized. The drive circuit of this invention is suitable for driving either piezoelectric or magneto-electromagnetic actuating mechanisms in a resonant system. The driver circuitry incorporates means for controlling output amplitude of the actuator/mass to a high degree of precision. The sensing of resonant frequency and amplitude is provided by the driven actuator mass itself, without the need for any independent or complex sensor circuits to provide feedback signals. This reduces resonant system complexities and eliminates several possible sources of error. The driver circuit of this invention continually energizes the actuator/mass at its natural resonance, even though the resonant system may have significant shifts in its resonant frequency over time, temperature, age or the like.